Hi Charleyjane93,
The voucher email should have been delivered to the email address you supplied VM when signing up 28 days after installation. Sometimes, due to miss-filtering by the email provider the email might end up in the spam folder.
You can either choose to wait up to three days for a response from a member of the Forum Team, who will happily look into the issue for you and submit a Voucher Exceptions Form. Once this form has been submitted your eligibility for the voucher will be checked. Alternatively, you can ring 150 (from a VM phone) or 0345 454 1111 from any other phone and speak to the Customer Care Team who can be reached from the main menu by selecting > option 4, followed by option 1.